You may be getting interim payments right now. After the OPM has your paperwork but before they have finished processing it, they make interim payments of 75% to 80% of your eventual annuity. During the interim period, they do not make FEHB or FEGLI deductions. After they finish their work, they send the actual annuity (with deductions) and they make up any amount that they owe you.

I hope that this is what happened. If so, bigger checks will be on their way.